An LC circuit, also called a tank circuit, resonant circuit, or tuned circuit, is an electric circuit comprising an inductive component, such as an inductor, represented by the letter L, and a capacitive component, such as a capacitor, represented by the letter C, connected in parallel, for example. The circuit can act as an electrical resonator storing energy oscillating at the circuit's resonant frequency. LC circuits are used either for generating signals at a particular frequency, or selecting a signal at a particular frequency from a more complex signal. They are key components in many electronic devices, particularly in radio equipment, used in circuits such as filters, oscillators, tuners, radio frequency (RF) modulators, sine wave generators and frequency mixers. An LC oscillator is a type of oscillator where an LC tank circuit is used for giving the required positive feedback for sustaining the oscillations. According to the Barkhausen criterion for sustained oscillations, a circuit will sustain stable oscillations only for frequencies at which the loop gain of the system is equal to or greater than 1 and the phase shift between input and output is 0 or an integral multiple of 2π.
For operating a tank circuit, the capacitor is first charged to a voltage V which is the voltage source. After this, the capacitor is allowed to discharge through the inductor. The voltage across the capacitor start decreasing and the current through the inductor starts increasing. The increasing current creates an electromagnetic field around the coil (inductor) and when the capacitor is fully discharged the electrostatic energy stored in the capacitor is fully transferred into the coil as electro-magnetic field. With no more energy in the capacitor to sustain the current through the coil, the field around the coil starts to fall and the current through the coil tends to decrease. Due to electromagnetic induction, the inductor generates a back electromotive force equal to L(di/dt) in order oppose the change in current. This back electromotive force starts charging the capacitor again.
When the capacitor is fully charged, the energy once stored in the inductor as electro-magnetic field has been moved to the capacitor as electrostatic field. Then the capacitor starts discharging again and the cycle is repeated. This cyclic transfer of energy between the capacitor and inductor is the reason behind the production of oscillations in the tank circuit.
Most of the LC oscillators can be categorised in the following oscillator types: Pierce oscillators, Clapp oscillators, Colpitts oscillators, transformer based oscillators, three-point oscillators (negative transconductance gm) and Hartley oscillators. Today state of the art complementary metal-oxide-semiconductor (CMOS) design oscillators use negative gm based oscillators also known as differential cross-coupled negative gm oscillators.
However, the above mentioned oscillator types have some limitations. More specifically, the above mentioned oscillator types are unsuited for frequencies below GHz and/or low power/low cost transmitter applications and do not provide a particularly good output signal amplitude vs direct current power ratio.
It is an object of the present invention to overcome at least some of the problems identified above related to oscillators.